The present invention relates to a method of etching, particularly to a method of continuing stable etching.
It has been known to carry out selective (preferential) etching to silicon with a mixed acid containing hydrofluoric acid, nitric acid and acetic acid. As the mixed acid, there can be mentioned an etching solution containing these acid components at 1:3:12 in volume as developed by Dash for testing drawbacks in silicon materials. Moreover, it is now noted that the etching solution containing the acid components at 1:3:8 in volume is applicable to isotropic selective etching because the etching rate of the solution depends on the concentration of impurity contained in the crystal.
However, for such a selective etching solution, the etching properties tend to be changed and the etching are not well controlled. Therefore, Muraoka et al. proposed a method of controlling the selectivity by adding aqueous hydrogen peroxide to the etching solution (Japanese Patent Publication SHO 49-45035).
However, even by the Muraoka's method, etching reaction does not occur in the least in case of three-dimensional fine processing for intelligent sensors represented by the process for fabrication of acceleration sensors. Namely, the selective etching based on the above-mentioned method can not be applied to such three-dimensional fine processing.
Accordingly, the conventional etching method as described above can not be used for such selective etching as to satisfy the properties required for performing stable three-dimensional fine processing.